The present available smallest transformer for a primary voltage of 220 volts and 50 H.sub.z has a structue height of 26 mm. A smaller structure height is not economical, because to arrive at the necessary number of windings, awire diameter of less than 0.05 mm would be required.
The standard sizes of transformer laminations which limits the winding space also tend to limit possible size reduction in transformers. This in turn increases the danger of voltage breakdown between the primary and the secondary winding. A special insulation between the windings which has a dielectric strength up to about 2500 volts cannot be installed because of space limitations. The maximum power output of such a small transformer with a standard core EI 30a is about 1.2 VA with an efficiency of about 42%.
The known transformers have bad heat train characteristics due to their compact construction which is aggravated by having the primary and secondary winding wound on top of each other. The heat generated in one winding is directly transferred to the other winding. Furthermore, due to the limited winding space given by the laminations, the specific current in relation to the winding area is limited, so that the temperature conditions with increase of of power quickly become worse.
In particular for small transformers which should be suitable for installation on printed circuit boards, it is very desirable that they can be potted into a cup shaped housing. Mounting transformers by solder points on printed circuit boards, caused from powers of about 10 VA on, strong mechanical stresses and special clamping elements become necessary. Furthermore, the structure height of such transformers which is usually around 40 to 60 mm is excessive because standard construction heights for printed circuit boards are 12, 15, 18, and 22 mm. For contemporary transformers with acceptable efficiencies, such construction heights cannot be realized. Furthermore, the compact construction of the transformers causes a too large weight per area ratio. This weight could be reduced by a small amount if the potting of the transformer is omitted. In this case however the windings would be exposed to all outside influences such as humidity, acid mist, oxidation, etc. Furthermore, the danger of receiving an electric shock is increased since components under voltage can easily be touched. The reason for the above outlined disadvantages are caused mostly by the fact that the primary winding and the secondary winding are wound on top of each other on the center leg of the contemporary three-legged transformer. Furthermore, with this configuration, a high capacitive and inductive action exists between the primary and secondary winding. The capacitive coupling can be prevented with the insertion of a conductive foil between the two windings. If such a screen is inserted the capacity between primary and foil on one side and the capacity between foil and secondary winding on the other side will become high. At the same time the disadvantage of relative high leakage current over the foil can occur when one winding burns through.
Having the primary and secondary winding on top of each other and directly coupled with each other, in the known three-legged configuration, super-imposed high frequencies will be transferred between the two windings. This would be impossible if the coupling between the windings would go exclusively through the iron core. The iron core is not suitable for depolarization by high frequency currents. To make the transferred high frequency harmless on the secondary side, to protect for instance rectifiers, additional components are necessary such as capacitors across the rectifiers. Thus the sensitivity to malfunctioning will be increased in network systems.
The current pulse when switched can, depending on the switching moment and the phase of the primary current, be a multiple of the average current. The safety margin on the primary winding therefore is difficult to ascertain. Furthermore, the safety precautions can only be to a limited degree so that they are not very effective. By secondary rectification the rectifier has to be protected by a resistance in series which in turn will increase the number of potential malfunctioning components.
Another disadvantageous property of contemporary transfomers occurs at short circuit. If the transformer is not able to carry short circuit currents, it will burn through. To make it capable of carrying short circuit currents some components like thermal switches, non-linear resistors, etc., must be installed, which in turn will again increase the sensitivity for malfunction, or more powerful transformers of larger dimensions and weight must be used. Secondary voltage regulation is possible with contemporary transformers by changing the primary voltage. For this high power regulation resistors or other complicated mechanical means must be employed to change the primary voltage.